This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
With the development of wireless communications, various advanced technologies have been proposed, including such as hybrid Frequency Shift Keying-Quadrature Amplitude Modulation (FSK-QAM).
The hybrid FSK-QAM technology, which combines FSK and QAM, has been studied in some literatures with different names. Theoretical analysis and simulation results show that, for modest code rates in low signal-to-noise ratio regions, the normalized throughputs of the hybrid FSK-QAM scheme are close to those of the traditional QAM or FSK scheme.
Instead of modulating each group of n (n>1) bits on a single frequency tone using the ordinary 2n-ary QAM scheme, the hybrid FSK-QAM scheme requires splitting each group of n bits into a subgroup consisting of k bits and a subgroup consisting of q=n−k bits. According to the subgroup of k bits, an active frequency tone is selected among 2k candidates through the use of 2k-ary FSK scheme. On the other hand, 2q-ary QAM may be performed on the subgroup of q bits to obtain a QAM symbol. Thus, the FSK scheme and the QAM scheme are combined in such a manner that a 2q-ary QAM symbol is carried by a selected one of 2k frequency tone candidates.
By way of illustration, FIG. 1 depicts an example of a hybrid FSK-QAM scheme, where a subgroup of k=2 bits from a group of n=4 bits is used for selecting an active frequency tone from 2k=4 candidates (f0, f1, f2 and f3), the other subgroup of q=2 bits from the group of 4 bits is used for selecting a QAM symbol from 2q=4 QAM constellation points (q0, q1, q2 and q3), and the selected QAM symbol is carried on the selected frequency tone. FIG. 2 further illustrates a result of performing the above-described bit splitting operation, where each group of n bits are split into a subgroup of k bits and a subgroup of q bits.
Another advanced technology is transmit diversity. As a robust and mature Multi-Input Multi-Output (MIMO) transmission technology, the transmit diversity scheme is selectable from various MIMO transmission modes for guaranteeing a reliable reception performance by providing redundant transmissions through multiple antennas deployed at the transmission side. By way of example, for all downlink control channels, including Physical Downlink Control CHannel (PDCCH), Physical Hybrid Automatic repeat request Indicator CHannel (PHICH) and Physical Control Format Indicator CHannel (PCFICH), the transmit diversity scheme may be used to guarantee the reception reliability. For Physical Downlink Shared CHannel (PDSCH), the transmit diversity scheme may also be selected as a fallback mode to guarantee the reliability. This is particularly true for terminal devices which are located at the edge of cells and thus undergo heavy inter-cell interference.
Specific transmit diversity schemes defined by the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) are spatial-frequency block code (SFBC) scheme for 2 antenna ports and SFBC-frequency switching transmit diversity (FSTD) for 4 antenna ports, respectively.
By way of illustration, FIG. 3 schematically depicts an exemplary implementation of the SFBC scheme for 2 antenna ports. As illustrated, initially, through the layer mapping block, each pair of consecutive modulation symbols d(2i) and d(2i+1) are mapped onto two layers as x(0)(i)=d(2i) and x(1)(i)=d(2i+1). Then, at the transmit diversity precoding block, Alamouti coding is applied to x(0)(i) and x(1)(i) so that the coded symbols are obtained as follows,
      [                                                      x                              (                0                )                                      ⁡                          (              i              )                                                                          x                              (                1                )                                      ⁡                          (              i              )                                                                        -                                          (                                                      x                                          (                      1                      )                                                        ⁡                                      (                    i                    )                                                  )                            *                                                                          (                                                x                                      (                    0                    )                                                  ⁡                                  (                  i                  )                                            )                        *                                ]    ,where the symbol * denotes the conjugate operation, the first row of consecutive coded symbols x(0)(i) and x(1)(i) are to be transmitted through antenna port 0, and the second row of consecutive coded symbols −(x(1)(i)* and (x(0)(i))* are to be transmitted through antenna port 1.
Next, the coded symbols pass through the Resource Element (RE) mapping block, where the first column of coded symbols x(0)(i) and −(x(1)(i))* are allocated to an RE (denoted as RE(k)) and the second column of coded symbols x(1)(i) and (x(0)(i))* are allocated to another RE (denoted as RE(k+1)).
The existing hybrid FSK-QAM scheme cannot be used with the existing transmit diversity scheme, because the application of the former scheme prevents the latter scheme from achieving a satisfactory performance.